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In my experience there are many failures due to CPE, and also wrongly configured DNS servers, but many more because PMTUD filtering (on purpose or by mistake, such as load balancers based in ECMP). Some ISPs, such as 1&1, even being aware of the problem, just ignore it, so anyone having a lower MTU can't access any of their customers web sites. Really bad and ugly, it took me almost 2 years to get a response from them and still waiting them for sorting it out ... Furthermore, happy eye-balls, in my opinion, is not doing a good job, so this means less IPv6 traffic that indeed we could have. Fortunately there is some ongoing work to update it (https://tools.ietf.org/html/draft-ietf-v6ops-rfc6555bis-01), even I think we should consider dropping it, as in IPv4 we don't have a similar thing, and this means when there are some issues in a network, the operators quickly realize about that, but unfortunately, happy eye-balls is hiding this in the majority of the cases.